The subject invention relates to films of fluorocarbon copolymers of ethylene copolymerized with tetrafluoroethylene or chlorotrifluoroethylene, and optionally other comonomers, and a process for their preparation.
Ethylene/tetrafluoroethylene copolymers of a mole ratio of about 1:1 are potentially useful in many applications because of their outstanding chemical and electrical resistance properties. Unfortunately, their tensile properties, and specifically their low modulus of elasticity (the films are too soft) and F5 value, i.e., the stress required to stretch the film 5 percent at a selected temperature, prevents the copolymer films from being economically converted into products by conventional high-speed machinery. Also, unfortunately, these copolymer films do not resist creep well enough over long periods of time to carry moderate loads without excessive deformation. Furthermore, while the copolymers have high melting points (of about 270.degree. C.), the tensile properties of these copolymer films are relatively low to begin with and decline further at temperatures well below the melting point.
Copolymers of ethylene/chlorotrifluoroethylene are disclosed in U.S. Pat. No. 2,392,378 to Hanford, but such copolymers melt at 240.degree. C. or below. A procedure for preparing somewhat higher melting copolymers of these monomers is disclosed in European Polymers Journal, Vol. 3, pages 129-144 (1967), but even these higher melting copolymers cannot be used in high temperature applications since they suffer from the same disadvantage as the ethylene/tetrafluoroethylene copolymers. Films of these copolymers, copolymerized with an adjuvant amount of a third comonomer, are disclosed in U.S. Pat. No. 3,624,250, and can be irradiated as in U.S. Pat. No. Re. 28,628, both to Carlson, however, no mention is made of the benefits achieved when films of those copolymers are uniaxially stretched.